MST is based largely on a social-ecological theory of development and behavior. Key features of this theory are that behavior is multidetermined and bidirectional in nature. Children are modeled within multiple systems (family, peer, school, and neighborhood) that have reciprocal influences on behavior. In practice, the MST model also draws on strategic and structural theories of family therapy. Both theories conceptualize problem behavior as being multidetermined, with interventions designed to address multiple levels of client needs. MST was validated initially in several randomized trials as an effective treatment for youth violence and chronic offending; however, given the overlap between the dynamics of youth substance abuse and delinquency, MST might reasonably be expected to impact adolescent substance use as well. As reviewed by Sheidow and Henggeler, findings from the early clinical trials of MST with serious juvenile offenders supported the potential for MST's positive impact on adolescent substance use. This Special Issue presents several articles that represent the direction of recent and ongoing substance-related research conducted at the Family Services Research Center (FSRC), Medical University of South Carolina, and in collaboration with colleagues nationally and internationally. The overarching mission of the FSRC is to increase the effectiveness of mental health and substance abuse services for children, adolescents, and their families by conducting rigorous research on the development, validation, and dissemination of clinically effective and cost-effective mental health and substance abuse services for youth who present serious clinical problems, as well as their families. 31 references